1. Field of the Invention
This invention relates to an apparatus and method for performing capillary electrophoresis, and more specifically to a thermal control system for a capillary electrophoresis instrument.
2. Description of the Prior Art
Capillary electrophoresis (CE) is a chemistry separation technique which utilizes the differences in solute electrophoretic velocity to isolate the various components of a sample. FIG. 1 depicts a typical CE apparatus. A high intensity electrical field supplied by high voltage power supply 10 is applied across a teflon, glass, or quartz separation capillary tube 12 of narrow inside diameter (5 to 400 micrometers) containing an electrolytic buffer solution. For an uncoated, open capillary tube, the presence of the electrical field imparts motion to charged and uncharged moieties present in the buffer through two mechanisms: electro-osmotic (endo-osmotic) flow and electrophoretic force. Flow of buffer (or sample from sample vial 14) through capillary 12 is detected by a detector 16.
Electro-osmotic flow is the bulk flow of buffer from a first buffer vial 18 to a second buffer vial 19 through capillary 12 due to the shearing movement of a diffuse layer of cations past a more firmly held, dense layer, interacting with integral, anionic groups of the capillary wall. Factors which influence the velocity of electro-osmotic flow are: electrical field strength; buffer dielectric constant; zeta potential (the electrical potential existing between diffuse and compact cationic layers); and buffer viscosity (which is dependent on bulk properties of the buffer and the temperature of the buffer). For electro-osmotically driven, packed capillary, reverse phase chromatography applications, solvents of use are any normally used solvent for standard reverse phase liquid chromatography.
Electrophoretic force is the force applied to charged particles residing in an electrical field, and neutral or uncharged molecules are not affected. Positively charged molecules (cations) migrate towards the cathode while negatively charged molecules (anions) move towards the anode. Factors controlling solute electrophoretic velocity are: molecular charge; electrical field strength; viscosity of the migration media; and solute molecular geometric factors.
The net velocity at which a solute travels in an uncoated, open capillary tube during CE is the vector sum of the electro-osmotic and electrophoretic velocities. Buffer viscosity plays a significant role for both of these phenomenon. Both electrophoretic and electro-osmotic velocities are inversely proportional to buffer viscosity, thus affecting the net migration velocity for all solutes.
When an electrical field is applied to a capillary which contains buffer, joule heating occurs. Accordingly the temperature of the buffer within the capillary increases until a steady state of heat exchange between the capillary and its surrounding environment is achieved. Consequently the ultimate buffer temperature is dependent upon the ambient temperature surrounding the capillary. Because of the temperature dependence of viscosity, the mobility of a solute in a given buffer within a given capillary in a given electrical field is largely determined by ambient temperature. For temperatures between 15.degree. to 30.degree. C., a 1.degree. C. temperature increase results in an approximate 2 percent decrease in viscosity, increasing solute net velocity by 2 percent.
As is the case in many chromatographic techniques, solute identity is linked to migration time and velocity. For one form of CE known as capillary zone electrophoresis, samples are loaded into the capillary as a slug or plug. The latter may be achieved by application of an electrical field or some hydrodynamic force (vacuum or pressure head). An electrical field is then applied and the solutes migrate, as bands, down the capillary at their respective net velocities. Differences among these velocities create the primary mechanism for solute separation. These solute bands are then detected by monitoring a bulk property of the buffer such as refractive index, photometric absorbance, fluorescence, electrical conductivity, or thermal conductivity. The time period extending from the initiation of the separatory process to the point of solute detection is termed the migration time. The net velocity is determined using the migration time and the distance traveled by the solute.
Because of the high efficiencies achieved in capillary electrophoresis, it is not uncommon to see peak widths as narrow as two to three seconds. For complex solute matrices, multiple peaks may be separated by as little as two to three seconds in migration time. Consequently, a twenty minute CE run in which the temperature has changed by 0.1.degree. C. can experience changes in migration time by as much as 2.4 seconds, possibly causing improper solute identification. Thus, efficient temperature regulation is required.
In the prior art, a capillary tube 12 as used in an electrophoresis instrument is supported in a variety of ways, depending on whether tube 12 is to be cooled by air, by liquid, or by metal plates in contact with the capillary tube. Cooling of tube 12 is important since the electro-phoresis process subjects the capillary tube to a very high voltage which causes joule heating in the capillary tube. It is important to maintain the temperature of the tube at a stable predetermined temperature so as to be able to make measurements at a known temperature. Various schemes have been suggested for supporting and cooling the capillary tube, all of which have significant disadvantages and many of which are not suitable for air cooling purposes.
Prior art electrophoresis and similar spectrographic instruments typically include an optical path as shown in FIG. 2, which includes two light sources 22, 24 each of which provides a different spectra. Typically one light source 22 is a deuterium (D.sub.2) source and the second light source 24 is a tungsten (W) light source. A movable shutter 26 is provided in front of light sources 22, 24 so as to switch in light source 22 or light source 24 depending on which spectra is desired. A light beam 28 from either light source is then passed through baffles 29 onto a concave holographic grating 30 or similar diffraction device, and then is focused into beam splitter 32 through baffles 33.
Beam splitter 32 in one form in the prior art is a short length of optical fibers. In the typical prior art instrument, a portion of the light transmitted to some of the optical fibers emerges from the beam splitter 32 at reference arm 34 and is sent via window 36 to a reference photodetector 38 which detects the reference light beam for purposes of comparison. The remainder of the light transmitted through beam splitter 32 is transmitted through a longer length of optical fibers to sample end 40 of the beam splitter and is focused using a lens 42 into sample cell 44 in which the sample is held. The portion of the light which passes through sample cell 44 and the sample therein is then directed onto a second (sample) photodetector 46 through window 48. The first and second photodetectors 38, 46 are matched substrate photodetectors, i.e. cut from the same piece of crystal or other photodetecting material, so as to have matching thermal properties. Also shown is monochromator casing 50. The dual beam approach compensates for fluctuations and the changes in intensity of the light source level, as well as any changes in intensity in the propagation of light.
For the purpose of remote detection in which only the sample arm is elongated, this prior art system has several disadvantages. Since reference photodetector 38 and sample photodetector 46 would be widely separated, they are subject to different amounts of heat due to their different locations. Thereby the problem of dark current i.e., drift caused by unequal heating, is significant, resulting in less precise measurements. Also, if the sample arm 40 of beam splitter 32 (i.e., that portion of the optical path which leads to the sample) is mechanically flexed, this flexing distorts the optical path through the optical fibers in sample arm 40, resulting in more or less light reaching sample cell 44. Since the portion of the light beam which reaches reference detector 38 is not so distorted, this causes a difference between the reference light beam and the sample light beam. Thereby, the prior art system is deficient because the common path of propagation is not maintained to the sample 46 and reference photodetectors 38.
Another significant problem with prior art electrophoresis instruments is the relative difficulty of controlling the temperature of the sample inside the capillary tube. As discussed above, capillary tubes are typically cooled by forced air or circulating liquid or by placing the capillary tube between metal radiator plates. The object is to cool and/or heat the capillary to a particular target temperature. Typically, the temperature control of the capillary tube in the prior art is performed by monitoring the temperature of the media surrounding the capillary tube. This process is problematic in that a thermal dam occurs at the interface between the media surrounding the capillary tube and the capillary tube itself. That is, thermal transfer is inhibited across this boundary, and therefore the temperature of the media surrounding the capillary tube is not exactly the same as that of the capillary tube itself.
As discussed above, electro-osmotic flow is the bulk flow of a solution to the capillary tube under high voltage which occurs in most forms of capillary electrophoresis in which the interior wall of the capillary tube has not been treated. It is well known that solutes move through the capillary tubing under the influence of the applied electric field at a net velocity equal to the vector sum of the electrophoretic velocity and the electro-osmotic velocity. Thus a cation, neglecting any solute-wall interactions, will have two mobilities or velocities in the same direction and thus will tend to move through the tubing relatively quickly. An anion will have an electrophoretic velocity which is the vector opposite direction of the electro-osmotic velocity and thus will tend to move through the capillary tubing relatively slowly. A non-charged species i.e., a neutral species, will have no electrophoretic velocity at all and thus can be used to measure the electro-osmotic velocity of the system. Typically amides or some other neutral species are used to measure electro-osmotic velocity. These materials are typically known as neutral markers. The term neutral marker refers to the fact that in the buffer of interest, the neutral marker solute has no electrical charge.
In the prior art, electro-osmotic flow is determined by introduction of a neutral marker and then observing at one particular wavelength the flow of the neutral marker through the system to identify when the neutral marker passes the detector. This process works well with very simple sample combinations, where no other solutes co-migrate with the neutral marker. If however other compounds present in the sample combination are also neutral, this complicates the process of detecting the neutral marker.
It is also known to detect electro-osmotic flow without the use of a neutral marker. In one known process, the electro-osmotic flow is determined by the level of current stabilization when different buffer solutions having different specific conductivities were provided in the anode and buffer reservoirs. This process relies on the assumption that the system demonstrates a zeta potential and dielectric constant which is not seriously affected by the change in the electrolyte composition in the solutes. In another method, electro-osmotic flow is determined without the use of a neutral marker by observing continuously the weight of the material held in the cathode buffer reservoir. The volume transfer is then determined by dividing the change in mass of the cathode buffer reservoir by the density of the buffer. These last two methods are extremely time consuming and difficult and require significant manual intervention in addition to being of doubtful accuracy. Thus, there is a significant need for a method to determine the electro-osmotic flow by an automated process which can deal with complex sample combinations.